因为：
errno_t strcat_s(
char *strDestination,
size_t numberOfElements,
const char *strSource
);
而不是你想象的
char *strcat_s(
char *strDestination,
size_t numberOfElements,
const char *strSource
);
ms-help://MS.VSCC.v90/MS.MSDNQTR.v90.chs/dv_vccrt/html/0f2f9901-c5c5-480b-98bc-f8f690792fc0.htm
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C#
Visual C++
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Run-Time Library Reference
strcat_s, wcscat_s, _mbscat_s
Example See Also Send Feedback
Append a string. These are versions of strcat, wcscat, _mbscat with security enhancements as described in Security Enhancements in the CRT.
errno_t strcat_s(
char *strDestination,
size_t numberOfElements,
const char *strSource
);
errno_t wcscat_s(
wchar_t *strDestination,
size_t numberOfElements,
const wchar_t *strSource
);
errno_t _mbscat_s(
unsigned char *strDestination,
size_t numberOfElements,
const unsigned char *strSource
);
template <size_t size>
errno_t strcat_s(
char (&strDestination)[size],
const char *strSource
); // C++ only
template <size_t size>
errno_t wcscat_s(
wchar_t (&strDestination)[size],
const wchar_t *strSource
); // C++ only
template <size_t size>
errno_t _mbscat_s(
unsigned char (&strDestination)[size],
const unsigned char *strSource
); // C++ only
Parameters
strDestination
Null-terminated destination string buffer.
numberOfElements
Size of the destination string buffer.
strSource
Null-terminated source string buffer.
Return Value
Zero if successful; an error code on failure.
strDestination
numberOfElements
strSource
Return value
Contents of strDestination
NULL or unterminated
any
any
EINVAL
not modified
any
any
NULL
EINVAL
strDestination[0] set to 0
any
0, or too small
any
ERANGE
strDestination[0] set to 0
Remarks
The strcat_s function appends strSource to strDestination and terminates the resulting string with a null character. The initial character of strSource overwrites the terminating null character of strDestination. The behavior of strcat_s is undefined if the source and destination strings overlap.
Note that the second parameter is the total size of the buffer, not the remaining size:
Copy Code
char buf[16];
strcpy_s(buf, 16, "Start");
strcat_s(buf, 16, " End"); // Correct
strcat_s(buf, 16 – strlen(buf), " End"); // Incorrect
wcscat_s and _mbscat_s are wide-character and multibyte-character versions of strcat_s. The arguments and return value of wcscat_s are wide-character strings; those of _mbscat_s are multibyte-character strings. These three functions behave identically otherwise.
If strDestination is a null pointer, or is not null-terminated, or if strSource is a NULL pointer, or if the destination string is too small, the invalid parameter handler is invoked, as described in Parameter Validation . If execution is allowed to continue, these functions return EINVAL and set errno to EINVAL.
In C++, using these functions is simplified by template overloads; the overloads can infer buffer length automatically (eliminating the need to specify a size argument) and they can automatically replace older, non-secure functions with their newer, secure counterparts. For more information, see Secure Template Overloads.
The debug versions of these functions first fill the buffer with 0xFD. To disable this behavior, use _CrtSetDebugFillThreshold.
TCHAR.H routine
_UNICODE & _MBCS not defined
_MBCS defined
_UNICODE defined
_tcscat_s
strcat_s
_mbscat_s
wcscat_s
Requirements
Routine
Required header
strcat_s
<string.h>
wcscat_s
<string.h> or <wchar.h>
_mbscat_s
<mbstring.h>
For additional compatibility information, see Compatibility in the Introduction.
Example
See the code example in strcpy_s, wcscpy_s, _mbscpy_s.
.NET Framework Equivalent
System::String::Concat
See Also
Concepts
String Manipulation (CRT)
strncat, _strncat_l, wcsncat, wcsncat_l, _mbsncat _mbsncat_l
strncmp, wcsncmp, _mbsncmp, _mbsncmp_l
strncpy, _strncpy_l, wcsncpy, _wcsncpy_l, _mbsncpy, _mbsncpy_l
_strnicmp, _wcsnicmp, _mbsnicmp, _strnicmp_l, _wcsnicmp_l, _mbsnicmp_l
strrchr, wcsrchr, _mbsrchr, _mbsrchr_l
strspn, wcsspn, _mbsspn, _mbsspn_l
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